CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)	$ 72	$ 30	$ 210	$ 151
Impairment losses on investments	(45)	(70)	(92)	(227)
Income from continuing operations before income taxes	4,390	4,788	13,244	15,013
Tax effect	(1,116)	(1,203)	(3,299)	(3,824)
Income (loss) from continuing operations	3,274	3,585	9,945	11,189
Subtotal, pretax	(51)	178	488	999
Other Noninterest Expense	(3,022)	(2,887)	(9,574)	(8,499)
Citigroup's accumulated other comprehensive income (loss)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	167	410	628	1,160
Citigroup's accumulated other comprehensive income (loss) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	167	410	628	1,160
Total tax effect	(54)	(135)	(209)	(386)
Total amounts reclassified out of AOCI, pretax	221	545	837	1,546
Realized gains (losses) on investment securities
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	(46)	7	(136)	(19)
Realized gains (losses) on investment securities | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)	(72)	(30)	(210)	(151)
Impairment losses on investments	13	43	36	137
Income from continuing operations before income taxes	(59)	13	(174)	(14)
Tax effect	13	(6)	38	(5)
Income (loss) from continuing operations	(46)	7	(136)	(19)
Debt valuation adjustment (DVA)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	5	9	17	5
Debt valuation adjustment (DVA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)	7	12	23	8
Tax effect	(2)	(3)	(6)	(3)
Income (loss) from continuing operations	5	9	17	5
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	161	365	619	1,097
Cash flow hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Tax effect	(52)	(116)	(198)	(350)
Subtotal, pretax	213	481	817	1,447
Total amounts reclassified out of AOCI, after-tax	161	365	619	1,097
Cash flow hedges | (Gain) loss reclassified from AOCI | Interest rate contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax	212	480	814	1,444
Cash flow hedges | (Gain) loss reclassified from AOCI | Foreign exchange contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax	1	1	3	3
Benefit plan adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	48	38	141	100
Benefit plan adjustments | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	62	51	188	136
Total amounts reclassified out of AOCI, after-tax	48	38	141	100
Total tax effect	(14)	(13)	(47)	(36)
Prior service cost (benefit) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	(4)	(6)	(14)	(17)
Net actuarial loss | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	62	52	196	152
Curtailment/settlement impact | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense	4	5	6	1
Excluded component of fair value hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	(1)	(9)	(12)	(23)
Excluded component of fair value hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	(2)	(12)	(16)	(31)
Tax effect	1	3	4	8
Total amounts reclassified out of AOCI, after-tax	(1)	(9)	(12)	(23)
Foreign currency translation adjustment (CTA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	0	0	0	0
Tax effect	0	0	0	0
Total amounts reclassified out of AOCI, after-tax	0	0	0	0
Long-duration insurance contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	0		(1)
Long-duration insurance contracts | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	0	0	(1)	0
Tax effect	0	0	0	0
Total amounts reclassified out of AOCI, after-tax	$ 0	$ 0	$ (1)	$ 0